The present invention relates to photolithography and more specifically to the verification of a photomask for use in photolithography.
Photomasks and reticles (hereinafter, “photomasks” or “masks”) are used in photolithography with an exposure source to cast images in photoimageable films such as photoresists. Masks typically are partially transparent and partially opaque, often having a transparent quartz substrate with chrome metal patterns defining the opaque patterns thereon. The design of a mask is a complicated process. In order to correctly pattern the photoimageable film, the opaque features of the mask need to appear different from the patterns intended to be achieved in the photoimageable film. This arises because optical proximity effects upon the photolithographic exposure of nearby features must be compensated. As examples of optical proximity effects, lines on the mask can appear shorter when printed on the photoimageable film, and lines which are isolated (features which are not near other neighboring features) tend to shrink in width as they appear in the exposed photoimageable film. On the other hand, lines which are “nested”, that is, lines which lie between other neighboring lines, tend not to shrink as much as isolated lines.
Existing techniques for verifying the suitability of a mask or reticle for the photolithographic process can become computationally intensive, particularly when masks need to be verified for marginal exposure conditions, i.e., non-optimum focus and dose conditions. The process of verifying a mask involves determining whether the shapes on the mask will produce the desired exposure pattern in the photoimageable layer. As semiconductor chips can now contain several billion transistors per chip, the processing required to completely verify a photomask can take several days or even weeks to perform, even when significant computing resources are devoted to the task.